Ion/Molecule Attachment Reactions: Mass Spectrometry

Parametric Sensitivity in a Generalized Model for Atmospheric Pressure Chemical Ionization Reactions

“…Analytical response in atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) − and ion mobility spectrometry (IMS) at ambient pressure , is governed by gas phase reactions between analyte neutrals and reagent ions, often derived from proton clusters in positive polarity. Such reactions occur in a significant number of ion sources for MS , including corona discharges, , electrospray ionization, − dielectric barrier discharges, , and soft X-rays. − One stable and reliable source at ambient pressure, the beta emitter 63 Ni, was used in early API MS instruments and can be found in IMS drift tubes in use today. , The foundations for understanding gas phase ion–molecule reactions were developed and refined by the mid-1970s with a broad experimental record for mass spectrometry including chemical ionization MS − and studies with ionization detectors. − These reactions as shown in eq for reactions between hydrated protons (reagent) and substances M (analyte) commonly occur at elevated or ambient pressure with ions at thermal energies. Favorable interactions between M and H + (H 2 O) n form an energetic intermediate (MH + (H 2 O) n )* which undergoes loss of neutrals to form a protonated monomer, MH + (H 2 O) x : Residence times and collision frequencies are often sufficient to result in larger cluster ions such as proton bound dimers shown in eq . While proton bound trimers can form at subambient temperatures and are observed in mobility spectra, ion lifetimes are submillisecond at or above 25 °C and are not commonly seen in applications of IMS today with 1 to 10 ms time scale. , Only reactions with hydrated protons are considered in models developed here although recent studies have demonstrated that precursor ions including O 2 + can be utilized at high electric fields and reduced pressures …”